Refrigeration systems which utilize the compression, expansion, liquidification, and evaporation of a working fluid to effect cooling below atmospheric temperatures are ubiquitous, constituting the bulk of such systems. In most systems, the compression and expansion (often used to help facilitate liquidification and evaporation) are enabled by mechanical compressor/pumps. These systems though effective, are costly, energy intensive, and require significant maintenance. This is particularly felt in cryogenic applications where it is difficult to operate the mechanical pumps with in the cryogenic environment and passing the working fluid between the cryogenic and atmospheric environment entails significant cost in terms of complexity and performance.
For applications where the performance or weight/complexity costs cannot be tolerated, cryogenic systems often use adsorption pumps for refrigeration. Such pumps typically include an adsorbent, such as activated charcoal or zeolite, that captures gas from the working fluid. However, adsorption pumps are limited by the fact that the adsorbent can only adsorb or capture a finite amount of gas. Once the adsorbent has reached this point, the adsorbent, and more generally the adsorption pump, is no longer operational. Then, the adsorbed or captured gas must be desorbed, typically by heating the adsorbent. Heating the adsorbent in the required manner causes the entire refrigeration system to be heated to higher than desired temperatures often for several hours at a time, making the system unusable for applications which require continuous usage.
Some attempts have been made to address the limitations of adsorption pumps for cryogenic cooling. A typical approach is to have two pumps and two separate chambers containing working fluid, essentially building two refrigeration systems. The two systems are then alternated, whereby one is run while the other is desorbed. Each refrigerator is attached to a common cold plate via a heat switch, which connects or isolates the refrigerator from the cold plate. While this works, heat switches become less effective (especially when in the conductive state) as temperatures are lowered, limiting the performance of such refrigeration systems, negating many of the advantages of adsorption refrigerators. Secondly, all of this engineering must be done at the coldest, most sensitive stage of the system.